Despite mounting evidence that the family is an important mediator of development, two aspects of family life have not been analyzed in depth: the extended family and siblings. Callitrichid primates are characterized by extended family groups, strong emotional attachment between adult males and females, and cooperative infant care. Since the expression of these traits varies widely across family units in callitrichid primates, these species represent ideal models in which to investigate relationships between early experiences in the family and subsequent biobehavioral development. This project will test the general hypothesis that the psychobiological development of infants is not dependent solely on characteristics of the parents and the nature of their interaction with the infants, but rather it should be sensitive to structural and contextual characteristics of the wider family system. The first aim will be to examine infant rearing patterns as a function of inter-individual relationship quality, family demographics, and interactions between the allocation of parental and alloparental care. The second aim is to evaluate the effects of early infant care on psychological and somatic development, and to examine social factors likely to mediate these relationships. The third aim is to investigate whether variation in receipt of caregiving behavior by infants subsequently influences patterns of behavior in adulthood, including heterosexual pair bonding and parental styles. The quality and quantity of care provided to infant marmosets, and developmental outcomes will be examined as families grow from a single breeding pair to an extended family. The impact of family life will be assessed for major developmental milestones, including infant growth and social development, becoming integrated into the larger social group, reproductive and social maturation, pair bond formation and subsequent parental competency. These studies will provide strong tests of the generality of evolutionary models predicting biobehavioral development within the family.